Possible Existence of Ultra Fast Polarity Diffusion Process of ZnO under High Pressure

The effects of heating temperature and duration time on the process of ZnO crystallization under the pressure of 5 GPa are studied systemically by the methods of X-ray diffraction (XRD) and scanning electron microscopy (SEM). The experimental results show that the diffusion rate of ZnO increases greatly under high-pressure and the sample density higher than 99% of the theoretical value can be achieved after heated at 400 ℃ and 5 GPa for 0.5 h. With the increasing of the heating temperature, micro-cracks develop along crystal boundary in all samples, and eventually ZnO powder mixed with single crystals and poly-crystals forms above 500 ℃. The pale yellow crystallites with an average size of 100 m and the largest size more than 350 m was obtained at 850 ℃ after heating of 5 min. The sample morphology analysis shows that the reason for the ultra fast crystallization of ZnO under highpressure is possibly caused by a very quick diffusion rate of ZnO along certain direction. The crystal boundary along such direction moves very quickly under high-pressure until the neighboring micro-crystals merge into a single crystal in a short time, while the micro-crystals shrink in the other direction and cause micro-cracks along other crystal boundaries.